In a thermoset coating system, it is required to have a crosslinking mechanism in order to obtain the desirable film properties such as solvent resistance, weatherability, and mechanical strength. Melamine/formaldehyde and isocyanates are the two common classes of crosslinkers used for polyol resins in the current coating technology. However, there has been extensive research effort in the coating industry to explore alternatives for these two crosslinking systems in order to meet current demands related to safety regulations and coating performance. These demands are resulted from concerns over the toxic nature of the isocyanates and problems associated with the melamine/formaldehyde system such as formaldehyde evolution and poor acid-etch resistance. In the present invention, a new class of crosslinkers have been discovered which may meet such demands.
It has been disclosed in the literature that imines of xcex1-amino acid esters may undergo 1,3-dipolar cycloaddition on heating with dipolarophiles such as maleic anhydride and dimethyl acetylenedicarboxylate to form adducts with cyclic structures. The imines may also be prepared by reacting hydrochloride salts of amino acid esters with benzaldehyde in the presence of triethylamine. The preparation of a tribenzaldehyde by reacting cyanuric chloride with p-hydroxybenzaldehyde has also been disclosed. The prior art, however, does not teach the preparation of multifunctional imine-esters and their application as crosslinkers for thermoset coatings.
The present invention describes the preparation of multifunctional imine-esters and their application as novel crosslinkers for surface coatings. The imine-esters may be formulated with polyesters containing maleic anhydride residues to give enamel formulations. These enamels may be cured by a 1,3-dipolar cycloaddition mechanism to give carbon-carbon bonds and cyclic structure. Coatings thus prepared have advantages of low toxicity, no formaldehyde evolution, and improved performance.
In the present invention, it has been discovered that multifunctional imine-esters act as novel crosslinkers for coating applications. The imine-esters may be prepared by reacting a di- or tri-aldehyde with an aminoester such as glycine ethyl ester hydrochloride (GEE). As shown in the reaction schemes below, di(imine-ester) (1) may be prepared by reacting terephthalaldehyde with GEE; whereas, tri(imine-ester) (2) may be prepared by reacting GEE with tribenzaldehyde (3) which in turn was synthesized by reacting cyanuric chloride with p-hydroxybenzaldehyde. 
These imine-ester crosslinkers may then undergo 1,3-dipolar cycloaddition on heating with maleic anhydride moieties to give highly desired carbon-carbon bonds and cyclic structure. This novel crosslinking mechanism may be demonstrated by the reaction of di(imine-ester) (1) with dimethyl maleate as illustrated in the reaction scheme below. The di(imine-ester) is first transformed on heating into an azomethine ylid which may then be readily reacted with dimethyl maleate to give a di-substituted adduct with cyclic structure. 
In the present invention, the imine-ester crosslinkers may be formulated with a polyester resin containing maleic anhydride residues to give baking enamels. The enamels thus prepared may then be coated on a substrate and subsequently baked to give cured films. The coatings have been found to be substantially crosslinked as evidenced by the good solvent resistance. The crosslinkers of the present invention have advantages of low toxicity, no formaldehyde evolution, and formation of cyclic structures for improved coating performance.
Thus, in one embodiment of this invention there is provided a crosslinker composition comprising the reaction products of
(a) a compound with multifunctional aldehyde groups represented by the formula Zxe2x80x94[Rxe2x80x94CHO]n 
wherein Z is a multifunctional residue; R is Ar or alkyl, n is 2 or 3 and
(b) an amine with an electron withdrawing group at the xcex1 position represented by the formula H2Nxe2x80x94CH(R)xe2x80x94E.W.
wherein R is H or alkyl; E.W. is an electron withdrawing group selected from CO2R, CN, COR, Cl, Br, or I and the like.
The equivalent ratio of (b) to (a) is preferably about 0.8 to 1.5. A more preferred equivalent ratio is about 1.0 to 1.3, and the most preferred equivalent ratio is about 1.0 to 1.1.
In another embodiment of the present invention, there is provided a crosslinker composition comprising the reaction products of
(a) a compound with multifunctional xcex1-aminoester groups represented by the formula Zxe2x80x94[OOCCH(R)NH2]n 
wherein Z is diol, triol, or polyol residues and R is H, or alkyl, n is 2 or 3 and
(b) an aromatic or aliphatic aldehyde The equivalent ratio of (b) to (a) is preferably about 0.8 to 1.5. A more preferred equivalent ratio is about 1.0 to 1.3, and the most preferred equivalent ratio is about 1.0 to 1.1.
In yet another embodiment of the present invention there is provided an enamel composition, comprising
(a) about 50 to 90 weight % of a polyester, based on the total weight of (a) and (b), comprising the reaction products of
(1) about 40 to 65 mole % of a diol, based on the total moles of (1), (2), (3), and (4),
(2) about 0 to 20 mole % of a polyol, based on the total moles of (1), (2), (3), and (4),
(3) about 0 to 25 mole % of a diacid residue, based on the total moles of (1), (2), (3), and (4),
(4) about 10 to 60 mole % of an ethylenically unsaturated diacid residue, based on the total moles of (1), (2), (3), and (4),
(b) about 10 to 50 weight % of an imine-ester crosslinker of the present invention, based on the total weight of (a) and (b); and
(c) a sufficient amount of an organic solvent to reduce the enamel viscosity to an applicable level.
A preferred weight % of (a) is about 60 to 50 and (b) is about 20 to 40. A more preferred weight % of (a) is about 60 to 70 and (b) is about 30 to 40.
Preferred mole % of (1) is about 45 to 60, (2) is about 3 to 15, (3) is about 5 to 20, and (4) is about 15 to 50. A more preferred mole % of (1) is about 50 to 55, (2) is about 5 to 10, (3) is about 8 to 15, and (4) is about 20 to 40.
This invention may be further illustrated by the following examples of preferred embodiments thereof, although it will be understood that these examples are included merely for purposes of illustration and are not intended to limit the scope of the invention unless otherwise specifically indicated.